Re: [Qemu-devel] [PATCH v16 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG

[Tetsuo Handa]

Wei Wang wrote:
> > And even if we could remove balloon_lock, you still cannot use
> > __GFP_DIRECT_RECLAIM at xb_set_page(). I think you will need to use
> > "whether it is safe to wait" flag from
> > "[PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify()" .
> 
> Without the lock being held, why couldn't we use __GFP_DIRECT_RECLAIM at 
> xb_set_page()?

Because of dependency shown below.

leak_balloon()
  xb_set_page()
    xb_preload(GFP_KERNEL)
      kmalloc(GFP_KERNEL)
        __alloc_pages_may_oom()
          Takes oom_lock
          out_of_memory()
            blocking_notifier_call_chain()
              leak_balloon()
                xb_set_page()
                  xb_preload(GFP_KERNEL)
                    kmalloc(GFP_KERNEL)
                      __alloc_pages_may_oom()
                        Fails to take oom_lock and loop forever

By the way, is xb_set_page() safe?
Sleeping in the kernel with preemption disabled is a bug, isn't it?
__radix_tree_preload() returns 0 with preemption disabled upon success.
xb_preload() disables preemption if __radix_tree_preload() fails.
Then, kmalloc() is called with preemption disabled, isn't it?
But xb_set_page() calls xb_preload(GFP_KERNEL) which might sleep with
preemption disabled.